LeetCode1137. N-th Tribonacci Number

一、题目

The Tribonacci sequence Tn is defined as follows:

T0 = 0, T1 = 1, T2 = 1, and Tn+3 = Tn + Tn+1 + Tn+2 for n >= 0.

Given n, return the value of Tn.

Example 1:

Input: n = 4
Output: 4
Explanation:
T_3 = 0 + 1 + 1 = 2
T_4 = 1 + 1 + 2 = 4
Example 2:

Input: n = 25
Output: 1389537

Constraints:

0 <= n <= 37
The answer is guaranteed to fit within a 32-bit integer, ie. answer <= 2^31 - 1.

二、题解

class Solution {
public:
    int tribonacci(int n) {
        vector<int> dp(n+1);
        if(n == 0) return 0;
        else if(n == 1) return 1;
        else if(n == 2) return 1;
        dp[0] = 0;
        dp[1] = 1;
        dp[2] = 1;
        for(int i = 3;i <= n;i++){
            dp[i] = dp[i-1] + dp[i-2] + dp[i-3];
        }
        return dp[n];
    }
};